This invention relates to a process for manufacturing pure silica, and in particular, to such a process utilizing primarily industrial grade raw materials.
The need for purer silica powder is gaining importance in the electronics industry as 256 K DRAM and megabit chips are becoming mass produced. It is important that these chips be encapsulated to protect them from problems such as moisture. The term "encapsulated" as used here refers to a package which consists of micro-miniature circuit embedded in an encapsulating material. Generally the encapsulation includes resin blocks, silica and urethanes, for example. When the chips become very dense as in the case of fabricating very large scale integrated circuits (VLSI), the purity of silicon powder used to form the encapsulating material becomes extremely important. Pure silica is needed to encapsulate these chips which protects them from the environment. The relative purity of the powder is extremely important as impurities can change the electrical characteristics of the silica and create conduction paths. For example, carbon, iron, as well as other metallics within the silica powder can create conduction paths which produce short circuits within the electronic components being encapsulated. Thus, it is essential in producing silica products for use within the electronic industry, that the quantity of carbon and iron within the silica powder be maintained at a minimum. Conventional electronic grade silica used for encapsulation usually contains from about 200 to 300 ppm carbon. Furthermore, uranium and thorium, which produce alpha radiation, must be kept at a minimum as this radiation from isotopes of these elements is capable of damaging the memory functons causing so-called soft errors.
Not only must the purity of the silica powder be controlled, but also the size of the particles of the powder must be controlled. If the powder is too fine, i.e., mostly -20 microns, then excessive quantities or resin must be used in forming the encapsulating material.
It has heretofore been known that such pure silica powder can be formed by a sol-gel processby gelling, drying, milling and calcining a batch made from equal volumes of distilled tetraethylorthosilicate (hereinafter (TEOS), reagent-grade ethanol, and acidified distilled water.
The raw materials have to be subject to multiple distillations to purify the same and ultra-clean rooms required to carry out the sol-gel process.
Although relatively pure silica powder is obtained when using the above constituents, the relatively high cost of the raw materials used in the process, the relatively high level of carbon found within the silica powder, and the lack of any substantial quantity of +325 mesh particles, results in a relatively expensive and not totally commercially satisfactory product.